The design of molding equipment for glass forming machines, were oriented mainly to increase the life of the molding equipment and to control and regulate the heat transfer.
However, due to the increase in the speed of operation of the glass forming machines, and to the limitations in mold thickness and air flow to cool the molds, the next advances were carried out on the development of new materials and new concepts to optimize the mold cooling.
Recently, the tendency to diminish the weight of the articles and maintaining or increasing their mechanical strength, had pushed to the glass industry to design the molds with an improved thermal control, in order to get a better glass distribution and additionally, as it will be possible, produce a surface free of flaws. For that, the technological strategies were focused on more sophisticated molds, which were not practical or economical in the past.
At the present time, the mold design must consider, the following:
That the glass mechanical strength is in a direct relation to its surface quality, as the fracture mechanic theories have established; and such surface quality is mainly configured by the mold; PA1 That the glass thickness distribution depends strongly of the cavity mold temperature, which implies to provide an adequate cooling and mechanisms to get thermal stability; and, PA1 The design criteria used in the past, that is, machine speed, air cooling and space limitation, mold life, replacement and maintenance time, cost, etc. PA1 1. That in the heat transfer process analysis involving any phase transformation like for example from solid to liquid and liquid to solid, it is possible to consider the latent heat as an increased heat capacity in the temperature range around the phase transition. Such temperature range going to zero for pure substances and eutectics. PA1 2. That heat capacity plays a role of heat storage, similar to the capacitor effect in the electric circuits or to the tanks in the hydraulic systems. PA1 3. That in Control Theory terms, the capacity effect gives stability to the system in response to the changes produced by the inputs and outputs.
When the suitable glass temperature range is lost, appear problems in the process, i.e. if the molds work too hot, the fault may be sticking of the glass at some part of the mold interfering with release of the ware thus resulting in checking and deforming the ware. If the mold works too cold, produce a chill wrinkled surface, which avoids a proper blown or pressed up fully to shape, or may be checked in the process of shaping.
One of the main problems of the previous art molds is that the heat transfer between the molten glass and the mold is not uniform. This causes uneven temperature distribution in the mold cavity, producing cracks or checks of thermal origin in some sections of the formed articles and/or an uneven distribution of the glass in the walls of the articles.
Another present situation of the art is that, due to the high speed of operation of the glass forming machines, the molds tend to work hotter.
In order to dissipate the heat, a great number of cooling systems are already known, for example, in U.S. Pat. No. 3,666,433 issued to H. H. Nebelung et al., a blank mold is provided with a thermocouple generally intermediate to the upper and lower ends of one mold half section, and with the temperature sensitive portion thereof as close as possible to the mold cavity itself. The mold is air cooled and a control system which includes a damper valve for controlling the flow of cooling air in response to the output of the thermocouple.
Some other cooling systems such as those described in U.S. Pat. Nos. 3,888,647 of Breeden et al., 4,361,434 of Schneider, 4,388,099 of Hermening et al., 4,502,879 of Foster, 4,525,191 of Fenton and 4,578,104 of Jones, also shown different arrangements for molds cooling, wherein the main cooling fluid is air.
However, one of the main problems of the molds that are cooled with air, is that in order to effect a better heat transfer, it is necessary to increase the heat transfer area or increase the flow of the cooling air. In last case, the increase in the air flow is limited by the excessive noise.
Another arrangements that have been suggested and used as fluid cooling of molds are those that use liquid coolants like water such as those described in U.S. Pat. Nos. 3,887,350 of Jenkins, 4,142,884 of Jones, 4,313,751 of Torok, etc. In these cases, the heat transfer occurs directly through the mold wall and into the fluid.
However, because these water cooling systems are closed circuits, its necessary to use treated water to avoid scaling of the ducts. Then, these systems have been non-practical because its connection is carried out when they are installed on the machine.
Another disadvantage of these systems, is that the water must be maintained at pressure slightly above of the atmospheric pressure. Basically, if the water is maintained to be cold within the mold will be very difficult to heat the mold and also to control the heat transfer. If the water is heated to be compatible with the temperature of glass mold, it will increase the sealing pressure and will produce boiling, scale, water steam, sealing problems and reducing the coefficient of heat transfer.
To overcome the water cooling systems, the U.S. Pat. No. 3,224,860 describes a glass forming mold for dissipating heat which receives from molten glass. Said mold comprising wall means having a first surface portion which is adapted to be placed in heat receiving contact with the molten vitreous material and a heat radiating second surface portion which is exposed to a coolant when the first surface portion is in contact with the molten material. Said surface portions defining at least one sealable internal chamber. A substance preferably as lead, tin, lead or tin alloys, an alkali metal, an alkali metal salt, an alkali metal hydride or an alkali metal hydroxide, which is in liquid state at temperatures about 500.degree. C. is introduced into the internal chamber, so the heat is transferred through this substance to dissipate into the atmosphere.
Also, the U.S. Pat. No. 3,644,110 describes a tool involved in glass making, comprising in combination, wall means enclosing at least one internal chamber and accommodating therein a vaporized volatile heat-exchange medium which is evaporable in the chamber in one region thereof and as a result of the heating of the tool, moves to another cooler region where it becomes condensed, and is returned to the one region by capillary means accommodated in the chamber.
In the above cited patents, the proposed metals and salts are considered to be poor heat conductors in relation with gray iron and aluminum-bronze alloys, and are necessary closed circuits that include a cooling section resulting in a complex system.
Another patents that attempt to improve the heat transfer are those that increase the effective area of the mold to be cooled, for example, the U.S. Pat. No. 3,849,101 of Wythe et al., shows a mold cavity defining structure which has formed an internal chamber. This chamber is filled with a porous filler material which comprises generally spherical metal particles which have been sintered to one another by brazing, or other similar process, to improve heat conduction among themselves. Means are provided for passing a cooling fluid, such as air, to further improve the heat transfer.
The U.S. Pat. Nos. 4,009,017 and 4,082,527 of Jones, shows a system and method for transferring heat through a glass forming mold. In these cases, a forming mold and cooling means for removing heat from the forming mold are included. The cooling means comprising a cooling chamber through which a cooling fluid may be circulated and a fluidizable bed of solid particles within a cavity located between a part of the forming mold and the cooling chamber. Associated with the cooling means is a source of gas, preferably air, under pressure for fluidizing the fluidizable bed of solid particles.
In practice, the design of an adequate mold depends on the type of machine and the selected operation speed.
Taking in account the design criteria and the limitations of the previous art, the inventors of the present invention consider that its necessary to design a mold as a self closed system having a great capacity to support the process variations, a great facility to homogenize its temperature, as well as, a great capacity to withdraw heat from the glass.
The present invention is based on the concepts:
In our invention, this effect will impart thermal stability to the mold, to changes in heat input due to variations timing, glass gob thermal properties, and in heat output due to variations in environment temperatures, cooling air, etc.
Typical examples involving the solidification or melting are of considerable importance in many technical fields: the making of ice, the freezing of foods, or the solidification and melting of metals in casting processes. For example, in the temperature distribution in an ice layer on the surface of a liquid, the upper face is exposed to air at subfreezing temperature. Ice formation occurs progressively at the solid-liquid interface as a result of heat transfer through the ice to the cold air. Heat flows by convection from the water to ice, by conduction through the ice, and by convection to the sink. The ice layer is subcooled except for the interface in contact with the liquid, which is at the freezing point. A portion of the heat transferred to the sink is used to cool the liquid at the interface solid-liquid to the freezing point and to remove its latent heat of solidification. The other portion serves to subcool the ice.
In a wide class of conduction processes in nature and technology, the system becomes divided into two or more regions by the transition isotherms. These phase boundaries traverse the body when the process is not stationary.
Taking in account the concepts described above, the present invention is referred to a new mold and plunger design for the manufacture of glass articles or similar materials, which comprises a first body having an internal cavity configured in accordance with the external profile of the article or the most adequate profile according to the forming process and cycle. A second body in contact with the first body to absorb heat and to control the heat extraction of said first body maintaining a two-phase zone with heat conduction and convection in the liquid zone, heat conduction in a solid zone and a solid-liquid interface motion in agreement with the temperature changes within the mold; and a third body which complements the mold, forming the external part of same to release the heat from said second body.
Within this context, the heat conditions obtained in this new mold will result in a better glass distribution.
Additionally, with the use of the present invention, the mold operation stability is improved, allowing that said mold work at high or low production speeds.
As a consequence of the above, it is possible to obtain articles whose wall thickness is more uniform. This permits a lightweighting of the same. This weight loss has as a consequence a reduction in the consumption of glass, fuels, forming time, etc.